1. Field of the Invention
This invention relates to a method of forming a monolayer of particles, and to products formed thereby. It is particularly concerned with forming an ordered array of particles in a monolayer, which may be incorporated into a film. Films formed by the inventive method have anisotropic conductive pathways formed by ordered arrays of conductive particles, and are especially useful in interconnection technology in the electronics industry.
The invention is also useful in other fields of technology and may be applied to particles which are not electrically conductive.
2. Description of Related Technology
Anisotropically-conductive adhesives and the ordering of "magnetic holes" in ferrofluids is discussed in WO 95/20820, the disclosure of which is expressly incorporated herein by reference.
JP 62-127 194 of Fujikura Cable Works KK describes the production of anisotropic conductive solder sheets by forming an adhesive coating having a thickness of less than 10 micrometers on a support film, applying soft solder powder having a grain size of 10-50 micrometers onto the adhesive coating, and filling the spaces between granules of the solder with a plastic material. It is stated that the soft solder granules can be evenly dispersed in the plastic material on the film. However, application of particles onto an adhesive film to which the particles adhere on contact is not believed likely to achieve satisfactory dispersion or ordering of the particles in the plane of the film.
EP 0 691 660 A1 of Hitachi Chemical Co. Ltd. describes an anisotropically electro-conductive film material produced by adhering electro-conductive particles to an adhering layer formed on a support and fixing the particles therein, and then introducing a film-forming resin incompatible with the adhering material between the electro-conductive particles, the film material having electro-conductivity only in the film thickness direction via the electro-conductive particles uniformly dispersed in the plane direction. The particles may be arranged in a grid or zig zag pattern in the plane by means of a film, net or screen having holes therein ("screen"), through which the particles are fixed on the adhering layer. The particles and the screen may be electrostatically charged with different electric charges. However problems exist in the use of such a screen, including difficulty in producing and handling thin screens, and making the desired patterns of holes. An individual screen would be required for each pattern. Also it would be difficult to (1) ensure that all of the holes are filled by particles and (2) guard against clogging of at least some of the holes by the adhesive material. Removal of the screen may also cause disruption of the pattern. The use of electrostatic charging would be a complex procedure involving large electrical fields.
U.S. Pat. No. 5,221,417 (Basavanhally) describes the use of photolithographic masking and etching to form a matrix array of mutually isolated ferromagnetic elements. These elements are magnetized and a single layer of conductive ferromagnetic particles is adhered to an upper surface of each of the ferromagnetic elements, so that the conductive particles are in an array. The layer of particles is then contacted with a layer of soft adhesive polymer to cause penetration of the particles into the polymer. The adhesive polymer is then hardened to assure containment of the particles in the polymer. The adhesive polymer containing the conductive particles is used for interconnecting conductor arrays. However, it is believed that this technique may be used only with conductive particles which are ferromagnetic. Such particles may be difficult to obtain in specific shapes, sizes and types (e.g., monodisperse spheres.)
JP 3-95298 discloses a conductive and magnetic fluid composition comprising colloid ferromagnetic particles and conductive particles dispersed in a carrier organic solvent.
U.S. Pat. No. 4,737,112 discloses an anisotropically conductive composite layer medium comprising electrically conductive magnetic particles in a non-conductive matrix. The particles are aligned via the interaction of an applied magnetic field with the electrically conductive particles. The invention relies on the use of magnetic particles as the conductors, and so has no utility in the preparation of ordered arrays of non-magnetic and substantially non-magnetic particles or in the preparation of systems in which ordered arrays are transferred from one substrate to another.
In WO 95/20820, a composition is described which includes: (i) a ferrofluid of a colloidal suspension of ferromagnetic particles in a non-magnetic carrier liquid, and (ii) electrically-conductive particles having substantially uniform sizes and shapes, dispersed in the ferrofluid.
The average particle size of the electrically conductive particles is at least 10 times that of the colloidal ferromagnetic particles. The non-magnetic carrier liquid may be curable or non-curable. Examples of the liquid include a curable liquid composition, a mixture of a curable liquid composition and a liquid carrier in which the ferromagnetic particles have been suspended, or a non-curable carrier liquid, provided the electrically-conductive particles have a latent adhesive property.
In this application, a method of making an anisotropically-conductive bond between two sets of conductors is also described. The method includes applying to one set of conductors a layer of an adhesive composition of the composition so described; bringing a second set of conductors against the layer of adhesive composition; exposing the layer of adhesive composition to a substantially uniform magnetic field such that interaction between the ferrofluid and the electrically-conductive particles causes the electrically-conductive particles to form a regular pattern of particles each in electrical contact with an adjacent particle and/or with a conductor in one or both sets whereby conductive pathways are provided from one set of conductors to the other set, each pathway including one or more of the electrically-conductive particles; and curing the composition to maintain the pattern in position and to bond the conductors.
It may not however always be convenient to install a means for creating a magnetic field at the location of assembly of two sets of conductors. Therefore, in EP 757,407, the disclosure of which is incorporated herein by reference, other ways are described of achieving the benefits of the invention of the WO 95/20820.
The EP 757407 describes an anisotropically-conductive film or a substrate having a surface coated with an anisotropically-conductive coating. The film or coating is formed by solidifying a composition which includes a solidifiable ferrofluid composition and electrically-conductive particles dispersed in the ferrofluid. The ferrofluid includes a colloidal suspension of ferromagnetic particles in a non-magnetic carrier. The electrically-conductive particles having been arrayed in a non-random pattern by application of a substantially uniform magnetic field to the composition in a liquid state and have been locked in position by solidification of the composition.
EP 757407 also describes a solid-form anisotropically-conductive film or a substrate having a surface coated with a solid-form anisotropically-conductive coating the film or coating includes a composition containing colloidal ferromagnetic particles and electrically-conductive particles arrayed in a non-random pattern.
The term "ferromagnetic" as used herein includes ferrimagnetic materials such as ferrites.
The term "solidifiable" as used herein means capable of existing as a solid at ambient temperatures (e.g., temperatures less than about 40.degree. C., usually about 20-30.degree. C.). Solidifiable compositions include curable compositions which cure to solid form by heat treatment or otherwise. The word "solid" as used in EP 757407 and also herein means stable in shape and includes a gel or polymer network.
The inventions of WO 95/20820 and EP757407 are a significant breakthrough in the uniform dispersion of conductive particles and address the issue of particle aggregation and the consequences in fine pitch electronic interconnection [cf. U.S. Pat. No. 5,221,417 (Basanvanhally)]. However, the preparation of a curable particle-loaded ferrofluid adhesive composition compromises between the ferrofluid character of the composition, including high magnetization saturation and low viscosity at room temperature for rapid ordering of the particles, and the adhesive character of the composition, including the use of medium to high molecular weight systems having relatively high viscosity, to impart good mechanical properties and functionality to the cured adhesive.
Accordingly, it would be desirable to provide ways in which monolayers of dispersed or ordered arrays of particles may be prepared as well as films prepared therefrom which are easy, fast and employ readily available, easy to manufacture components and which allow for the reuseability/recovery of otherwise expensive and/or government-regulated materials.
It would also be desirable to prepare stable monolayers of particles and arrays of particles and films and prepared therefrom which are free or substantially free of ferromagnetic particles and which containing random and ordered arrays of particles with improved physical and performance characteristics, e.g., improved strength and/or adhesiveness as well as transparency or translucency, and the like.